Recently, rechargeable secondary batteries using alkali metal cations are receiving focused attention as a clean energy source. Particularly, secondary batteries using a lithium ion have relatively low energy density, but has excellent oxidation stability, excellent solubility with an electrolyte solution and processibility, and thus, are widely used as advanced electronic products such as a PC, a display, a smart phone, etc. and batteries for supplying energy of automobiles. However, they are expensive and have limited reserves, and thus, the supply is limited. In order to solve such a problem of lithium ion batteries, development of next generation secondary batteries using metal ions with very high energy density such as sodium(Na), potassion(K), calcium(Ca), etc. is being actively progressed.
Besides, sodium and potassium cations control ion transport and action potential of cells in a lipid bilayer as well as physiological metabolism of organism, thus performing physiologically very important functions such as controlling of neural signal transmission, muscle contraction, and heart function, etc. (see Non-Patent Documents 1 and 2).
Thus, if a chemical sensor or separation membrane capable of selectively detecting alkali metal ions to control or separate is developed, the battery functions of the above explained secondary batteries may be improved and the energy density may be optimized, and various physiological functions can be more efficiently controlled.
Meanwhile, for use as a chemical sensor of alkali or transition metals, organic molecules or polymer materials into which a functional group capable of detecting and capturing metal ions should be used as a substrate of the sensor. As the molecules exhibiting such a sensor function, crown ether (see Non-Patent Documents 3 to 5), calixarene (see Non-Patent Documents 6 to 8) or unimolecular derivatives of azobenzene (see Non-Patent Document 9) have been mainly reported. Particularly, among these functional molecules, azobenzene has advantages of causing minute optical changes by photoisomerization such as changes in birefringence, absorptivity of complex and absorption wavelength. Thus, in order to use such optical properties, azopolymers into which an azobenzene group is introduced have been widely studied (see Non-Patent Documents 10 to 12). However, despite the above-described various advantages, azo-based monomers and polymers exhibiting the functions for detecting alkali metal or transition metal ions, and the properties and applications of these materials as a sensor material have been scarcely studied.